Mulching tree leaves: an alternative to disposal

Zac Reicher and Glenn Hardebeck

Few things are more beautiful than the changing colors of tree
foliage in the fall. Unfortunately, enthusiasm for this autumn
splendor is tempered by the knowledge that homeowners, professional
grounds managers and golf course superintendents will soon face the
inevitable task of leaf disposal.

There's no question that you have to do something with leaves. Tree
leaves can shade the turf, robbing it of precious photosynthetic
activity in the late fall. Even thin layers of tree leaves trap
humidity at the turf surface and increase the chance of snow mold
during winter. Thicker layers of leaves can smother and completely
kill the turf. Removing the interference from fallen tree leaves also
allows your late season nitrogen applications to reach the turf more
effectively, and improves the efficacy of late-season broadleaf
herbicide applications. Therefore, for optimum turf health, it is
critical to remove the tree leaves, or at least break them up.

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Dwindling options
Years ago, we raked leaves into piles and either burned them, moved
them to the curb for garbage pick-up or hauled them to the local
landfill. Today, burning tree leaves is banned or at least restricted
in most municipalities. Yard-waste bans prevent us from hauling tree
leaves to landfills and garbage haulers often no longer take tree
leaves left on the curb. If you are lucky, you have your own compost
pile or a composting center where you can still dispose of leaves,
but this still requires a lot of labor and expense. Land-applying
leaves-spreading them on agricultural land and tilling them in-is
good method of disposal, but it is expensive.

The easiest and cheapest way to dispose of leaves is to mulch them
into the turf. This is not a new idea, but universities have only
recently compiled enough data to determine that tree-leaf mulching
has no long-term negative effects on the turf. Studies at Michigan
State, Cornell, Rutgers and Purdue have concluded that mulching tree
leaves is an excellent disposal method that does not harm healthy
turf.

The Purdue study
* Methods. At Purdue, we are just finishing up a 5-year study looking
at the effects of mulching maple leaves into turf. Every October
since 1994, we collected maple leaves, ran them through a garden
shredder to facilitate handling and applied them to a low-maintenance
perennial ryegrass stand.

We applied the leaves at 2,000 and 4,000 pounds per acre in a single
application and set aside check plots where we applied no leaves. We
selected these rates because a typical forested area will drop about
3,000 pounds of tree litter per acre each year, including twigs and
leaves. Our 4,000-pound rate was the equivalent of about 6 to 8
inches of piled leaves. After application to the plots, we
immediately mowed the leaves with a mulching mower.

Because we felt that the high carbon-to-nitrogen ratio (C:N) in the
tree leaves would eventually limit nitrogen in the turf stand, we
annually applied nitrogen using three different rates. Beginning in
the spring of 1995, we applied 1.3 or 2.6 pounds of nitrogen per
1,000 square feet per year, and left some plots unfertilized.
However, by summer 1996, the ryegrass receiving no nitrogen had
declined dramatically. Therefore, we adjusted the rates to 1.3, 2.6
and 3.9 pounds per 1,000 square feet per year.

The turf plots received regular irrigation to prevent drought stress,
and herbicide applications to limitbroadleaf and annual weeds. We
collected data monthly for clipping weights, visual quality and color
ratings, and annually evaluated the turf mat, soil pH and soil
nutrient concentrations. We also checked soil microbial biomass and
activity, and water infiltration.

* Results. We found that even the high rate of tree leaves had no
effect on turf visual quality, color or growth. Although we expected
tree leaves to tie up nitrogen in the soil, we saw no long-term
effects of tree leaves on turf growth regardless of the nitrogen rate
we applied (other than the original plots that received no nitrogen
at all). Other studies have shown short-term nitrogen depletion as
the tree leaves break down. However, we feel this would not be a
problem in most areas because fall fertilization of the turf will
almost certainly ensure adequate nitrogen for the grass plants.

We also suspected long-term nitrogen depletion would increase
diseases such as red thread, pink patch and dollar spot, but we saw
no increase in these diseases regardless of leaf or nitrogen
treatment. No differences in weed infestation occurred, regardless of
treatments.

No substantial buildup of leaf mat occurred in the soil, though you
might expect this to occur after many years of leaf mulching.
However, we did see an increase in microbial activity caused by
mulching the tree leaves, which apparently prevented the leaf mat
from accumulating.

The increased microbial activity is perhaps the most interesting
finding of this study. Increased soil microbial activity indicates
improved soil quality. Therefore, we expect that the heavy clay soils
on which many new subdivisions are now built should improve as the
trees mature and their leaves are mulched into the turf.

We observed no effect on nutrient availability or pH in the soil.
However, we were applying maple leaves with a basic pH to a fertile
silt-loam soil with significant buffering capacity. Theory and
practical experience tell us that if low-pH leaves, such as oak, are
mulched into a poorly buffered soil, such as sand, the pH may
decrease significantly after many years of leaf mulching. Of course,
corrective measures such as lime applications can counteract this.

But is it practical?
Professional turf managers tell us that mulching tree leaves saves
tremendous amounts of time for their crews, allowing them to take on
more important or profitable jobs. One person can mulch tree leaves
on an entire property where it might have taken a crew of four or
five to pick up the leaves. Mulching tree leaves minimizes expenses
for equipment such as large vacuums and specialized trailers or
trucks for hauling. It also eliminates disposal fees at compost sites
or landfills.

For some in-house operations, this might be a "no-brainer." However,
revenue is an issue that inevitably comes up with contractors
whenever a new labor-saving strategy is devised. Why should
maintenance contractors eliminate leaf pick-up and disposal when it
can be a good source of revenue? This is an important question, and
one that may have different answers, depending on the property and
the maintenance operation. Isn't mulching tree leaves with mulching
mowers still a leaf-disposal service? Your company could still offer
a leaf management service, but it could take the form of late-season
mulching rather than leaf pick-up and disposal.

Leaf mulching can even be an effective bidding tool, because an
overall maintenance program that includes fall leaf disposal could be
performed for less money if you employ leaf mulching. Further,
maintenance practices perceived as environmentally responsible hold
great weight with some clients, so this could be a way to portray
yourself as an "eco-friendly" operator.

Is tree leaf mulching for everyone? Probably not--every situation is
different. A heavily wooded lawn, already thinned from shade, might
not be able to withstand the extra abuse of mulching the tree leaves.
Homeowners set in their ways may not be comfortable with the idea of
mulching tree leaves when they've always been raked in the past. It
is not realistic to expect every property to adopt a leaf-mulching
program. However, it is reasonable to evaluate each account and
recommend mulching where appropriate. Saving time and money without
harming turf makes leaf mulching a winning strategy.

Dr. Zac Reicher is a turfgrass extension specialist and Glenn
Hardebeck is a research technician, both at Purdue University (West
Lafayette, Ind.).